1. Field of the Invention
The present invention relates to a chemical mechanical polishing apparatus used in manufacturing a semiconductor device. More particularly, the present invention relates to a polishing head, and to the retainer ring of a polishing head of such a chemical mechanical polishing apparatus.
2. Description of the Related Art
Increasing the integration of semiconductor devices has required sequentially depositing multiple layers on a wafer. Accordingly, the semiconductor manufacturing process must include steps for planarizing each layer formed on the semiconductor wafer. Chemical mechanical polishing (CMP) is a typical process used for this purpose. In fact, CMP is well-suited for use in connection with large-diameter wafers because CMP produces excellent uniformity in planarizing wide areas in addition to narrow ones.
The CMP process makes use of mechanical friction and a chemical agent for finely polishing a wafer surface, such as that comprising tungsten or an oxide. In the mechanical aspect of such polishing, a wafer is placed on a rotating polishing pad and is rotated while a predetermined is load applied thereto, whereby the wafer surface is polished by the friction created between the polishing pad and the wafer surface. In the chemical aspect of such polishing, the wafer surface is polished by a chemical polishing agent, referred to as slurry, supplied between the polishing pad and the wafer.
A conventional CMP apparatus will now be described in with reference to FIG. 1. The conventional CMP apparatus includes a base 100, polishing pads 2120a, 210b and 210c installed on the base 100, a load-cup 300 for loading/unloading wafers, and a head rotation unit 400 having a plurality of polishing heads 410a, 410b, and 410d for holding the wafers and fixedly rotating the same on the polishing pads 210a, 210b and 210c. 
In general, the CMP apparatus is provided with three polishing pads 210a, 210b and 210c so that a plurality of wafers can be processed in a short time. Each of the polishing pads 210a, 210b and 210c is closely fixed on a rotatable carousel (not shown). Pad conditioners 211a, 211b and 211c for controlling the surface states of the polishing pads 210a, 210b and 210c and slurry supplying arms 212a, 212b and 212c for supplying slurry to the surfaces of the polishing pads 210a, 210b and 210c are provided in the vicinity of the polishing pads 210a, 210b and 210c. 
Also, the load-cup 300 includes a circular pedestal 310 on which the wafers are placed. The bottom surfaces of the polishing heads 410a, 410b, 410c and 410d and the top surface of the pedestal 310 are washed at the load-cup 300, as will be described later in more detail.
The head rotation unit 400 includes four polishing heads 410a, 410b, 410c and 410d and four rotation shafts 420a, 420b, 420c and 420d. The polishing heads 410a, 410b, 410c and 410d hold wafers and apply a predetermined amount of pressure to the top surfaces of the polishing pads 210a, 210b, 210c and 210d. The rotation shafts 420a, 420b, 420c and 420d for rotating the polishing heads 410a, 410b, 410c and 410d, respectively, are mounted on a frame 401 of the head rotation unit 400. A driving mechanism for rotating the rotation shafts 420a, 420b, 420c and 420d is provided within the frame 401 of the head rotation unit 400. The head rotation unit 400 is supported by a rotary bearing 402 so as to be rotatable about the longitudinal axis of the rotary bearing 402.
The process performed by the CMP apparatus having the above-described configuration will now be described with reference to FIGS. 1 and 2. First, a wafer 10 transferred to the load-cup 300 by a wafer transfer apparatus (not shown) is placed on the surface of the pedestal 310 of the load-cup 300. Here, the wafer 10 is adhered by suction to the surface of the pedestal 310 so as not to move. Then, the wafer 10 is lifted by the pedestal 310 onto a polishing head 410 positioned above the pedestal 310. The wafer 10 is adhered by suction to the polishing head 410. The head rotation unit 400 is rotated to transfer the wafer 10 in such a state above the polishing pad 210a adjacent to the load-cup 300. Then, the polishing head 410 is lowered to tightly press the wafer 10 onto the polishing pad 210a. At this time, the polishing pad 210 a and the wafer 10 are rotated in the same direction while slurry is supplied therebetween, whereby the wafer 10 is polished. The wafer 10 is then transferred sequentially to the other polishing pads 210b and 210c and then to the load-cup 300 where it is placed on the pedestal 310. Thereafter, the wafer transfer apparatus transfers the wafer 10 placed on the pedestal 310 to a location outside the CMP apparatus.
Once the wafer 10 has been unloaded, the polishing head 410 descends towards the load-cup 300. In such a state, deionized water is sprayed to wash the bottom surface of the polishing head 410 and the top surface of the pedestal 310. When washing is completed, the polishing head 410 and the pedestal 310 are lifted again and a new wafer is transferred by the wafer transfer apparatus onto the pedestal 310.
Referring to FIGS. 3 and 4, in order to wash the bottom surface of the polishing head 410 and the top surface of the pedestal 310, the load cup 300 is provided with washing means comprising a first nozzle 331 and a second nozzle 332 for spraying deionized water within a washing basin 320 of the load-cup 300. The first nozzle 331 is oriented so as to spray deionized water toward the top surface of the pedestal 310 and the second nozzle 332 is oriented so as to spray deionized water toward a membrane 411 installed on the bottom surface of the polishing head 410. The membrane 411 allows a vacuum to act on the wafers and secure them to the polishing head 410. Three sets each of the first and second nozzles 331 and 332 are installed at equal angular intervals around the circumference of the pedestal 310. Three wafer aligners 340 for guiding wafers are installed within the washing basin 320 of the load-cup 300 at equal angular intervals around the circumference of the pedestal 310 to guide the wafers placed on the pedestal 310 into position.
The washing basin 320 is supported by a cylindrical support housing 350, and a flexible hose 336 for supplying deionized water to the first and second nozzles 331 and 332 is installed within the support housing 350. A washing fluid channel 337 for connecting the flexible hose 336 to the first and second nozzles 331 and 332 is provided within the washing basin 320.
A plurality of spray orifices 311 for spraying deionized water upwards are provided in the pedestal 310 for the purpose of washing the membrane 411. A lateral passageway 312 connected to the spray orifices 311 is provided in the pedestal 310. The lateral passageway 312 is connected to a vertical passageway 313 formed inside a tubular pedestal column 315 supporting the pedestal 310.
As described above, the load-cup 300 is responsible for washing the bottom surface of the polishing head 410 and the top surface of the pedestal 310 as well as for supporting wafers while they are loaded and unloaded onto and from the CMP apparatus. The washing step is very important in the CMP process. Contaminants such as slurry debris or polished silicon particles are unavoidably produced during the CMP process, and some of the contaminants may remain on the surface of the membrane 411 and/or the pedestal 310. The contaminants remaining on the surface of the membrane 411 and/or the pedestal 310 can generate micro-scratches on the surface of a wafer if the contaminants are transferred thereto when the wafer is loaded in the course of polishing. The micro-scratches may cause defects such as gate oxide leakage or gate line bridging in the semiconductor devices, which lowers the yield and reliability of the semiconductor devices. Thus, any contaminants remaining on the membrane 411 and/or the pedestal 310 must be removed by washing the same using deionized water.
However, such contaminants cannot be completely removed by the washing operation performed by the conventional CMP apparatus. This washing operation will now be described with reference to FIGS. 5 through 7.
FIG. 5 is a cross-sectional view of a polishing head 410 of the conventional CMP apparatus, FIG. 6 is a detailed diagram of a portion xe2x80x9cAxe2x80x9d of the polishing head encircled in FIG. 5, and FIG. 7 is a perspective view of a conventional retainer ring of the polishing head.
The polishing head 410 of the CMP apparatus holds a wafer thereto under a predetermined amount of pressure and rotates the wafer in such a state. More specifically, the wafer is held by a vacuum to the polishing head 410 while it is rotated. To this end, a vacuum line 419 is provided within the polishing head 410, and a membrane support plate 414 having a plurality of holes 415 communicating with the vacuum line 419 is installed at the bottom of the polishing head 410. A membrane pad 416 is fixed close to the bottom of the membrane support plate 414. The bottom of the membrane pad 416 and the outer surface of the membrane support plate 414 are surrounded by the membrane 411, which is made of a flexible material which comes into direct contact with wafers. The membrane 411 is fixed to the membrane support plate 414 by a membrane clamp 417. A retainer ring 412 for preventing wafers from deviating outwards during polishing is disposed at the lower outer edge of the polishing head 410, that is, at the perimeter of the membrane 411. Four purge holes 4121 are provided at the outer circumference of the retainer ring 412 at equal angular intervals. While a wafer is adhered to the membrane 411, air can enter/leave a small space 418, formed between the membrane support plate 414 and the retainer ring 412, via the purge holes 4121.
In the polishing head 410 having the structure described above, a narrow gap having a width (D) of about 0.254 mm is present between the membrane 411 and the retainer ring 412 so that the membrane 411 can be elevated with respect to the retainer ring 412 when a load is applied to a wafer. However, the slurry or contaminants produced during polishing are induced into the space 418 through the gap having the width D. The induced slurry or contaminants induced into the space 418 are not removed by the washing operation. In other words, the deionized water sprayed from the first and second nozzles 331 and 332 shown in FIG. 4 cannot wash the contaminants induced into the space 418 because of the directions in which they spray the water and because of the narrowness of the gap D. Also, as shown in FIG. 7, although four purge holes 4121 are provided in the retainer ring 412, the diameters thereof are at most 2 mm. Thus, the contaminants induced into the space 418 cannot be exhausted through the small purge holes 4121. Thus, the contaminants accumulate over time and solidify as moisture evaporates therefrom.
The solidified contaminants drop onto the surface of a polishing pad due to vertical movement of the membrane 411 or slight vibration of the polishing pad during polishing. The size of the contaminants which drop onto the surface of the polishing pad exceed several micrometers, whereby micro-scratches or even macro-scratches can be formed in the surface of a wafer.
As described above, contaminants such as polished silicon particles or slurry debris are not completely removed in the conventional CMP apparatus. Thus, the surfaces polished by the CMP apparatus can become scratched, thereby lowering the yield and reliability of semiconductor devices produced from the wafers polished by the CMP apparatus.
It is therefore an object of the present invention to provide a polishing head, and more specifically, a retainer ring of a polishing head, which facilitates the removal of contaminants, such as slurry debris, from within a polishing head of a CMP apparatus.
It is another object of the present invention to provide a CMP apparatus which can effectively wash away contaminants, such as slurry debris, from within a polishing head.
To achieve the first object, the present invention provides a retainer ring which includes an annular ring body, a plurality of screw holes provided at the top surface of the ring body for allowing the ring body to be fixed in place in the polishing head, and a plurality of contaminant outlets extending from the inner peripheral surface of the ring body to the outer surface of the ring body and configured to allow contaminants within the polishing head to be exhausted to the outside of the polishing head under the centrifugal force that is produced when the polishing head is rotated.
At least six of the contaminant outlets are provided at substantially equal angular intervals about the periphery of the ring body, and the sum of the widths of all of the inner openings of the contaminant outlets, as measured a in the circumferential direction of the ring body, is at least 30% of the inner circumference of the ring body.
To further facilitate the discharge of the contaminants, the inner openings defined at the inner surface of the ring body and the outer openings defined at the outer surface of the ring body by the contaminant outlets are in the form of horizontally elongated slots, and each outer opening is preferably wider than that of each inner opening.
Also, each of the contaminant outlets preferably consists of a plurality of inner holes and an outer hole which is joined to the plurality of inner holes. The bottom of the outer hole slopes downwardly toward the outer peripheral surface of the ring body.
Furthermore, the contaminant outlets may extend longitudinally at a predetermined angle with respect to the radial direction of the ring body in a direction opposite to the direction in which the polishing head rotates during the polishing operation.
The contaminant outlets double as purge holes through which air passes when the wafer is vacuum-chucked to the polishing head.
To achieve the second object, the present invention provides a chemical mechanical polishing (CMP) apparatus for planarizing the surface of a semiconductor wafer, the CMP apparatus includes not only a polishing head having a retainer ring provided with contaminant outlets extending from the inner peripheral surface of the ring body to the outer peripheral surface thereof, but also washing means for spraying deionized water radially through the contaminant outlets of the retainer ring toward an inner space formed in the polishing head.
The washing means may be in the form of at least three third nozzles spaced from one another along the inner circumference of the load-cup of the CMP apparatus. Furthermore, an annular deionized water supply line may be installed along the inner surface of the load-cup, and the third nozzles are disposed in the deionized water supply line. The number of third nozzles is preferably the same as the number of contaminant outlets.
According to the present invention, the contaminants, such as slurry debris, which potentially could scratch the surface of a wafer, can be effectively washed or exhausted from the polishing head, thereby reducing defects in the semiconductor devices produced due to scratching.